1. Field of the Invention
This invention relates to a vulcanizable acrylic rubber composition, and more particularly to a vulcanizable acrylic rubber composition that can produce vulcanized products having very good compression set and tensile strength even if the secondary vulcanization is omitted.
2. Description of the Prior Art
Heretofore, various acrylic rubber vulcanization systems using halogens at cross-linking sites have been known and have their own problems to be solved. That is, in the case of a vulcanization system based on a combination of polyvalent amines such as hexamethylenediamine carbamate and hexamethylenetetramine, and dibasic lead phosphite or metal oxide as an acid acceptor, the vulcanization speed is low and the compression set is poor when 2-chloroethyl vinyl ether is a cross-linking site, whereas when vinyl chloroacetate is a cross-linking site, the vulcanization speed is practically too high.
In the case of a vulcanization system based on a combination of fatty acid metal soap and sulfur or a sulfur donor, the vulcanization speed and the compression set are also poor when 2-chloroethyl vinyl ether is a cross-linking site, whereas when vinyl chloroacetate is a cross-linking site, the compression set is poor, though the vulcanization speed is high.
Furthermore, in the case of a vulcanization system based on a combination of a triazine derivative and a metal oxide [Japanese Patent Publication No. 50-15815 and Japanese Patent Application Kokai (Laid-open) No. 51-58451], acrylic rubber using 2-chloroethyl vinyl ether as a cross-linking site has a low vulcanization speed and a short Mooney scorch time, whereas acrylic rubber using vinyl chloroacetate as a cross-linking site has too high vulcanization speed to the contrary.
Besides, a vulcanization system based on a combination of trithiocyanuric acid and a zinc thiocarbamate derivative or thiuram sulfide are also known (Japanese Patent Publication No. 49-13215), and improves the disadvantage of so far proposed, vulcanization systems based on a combination of soap and sulfur, i.e., inconvenience in attaining a highest curing state by rapid curing, though the proposed vulcanization systems can meet most of the objects. It is said that a composition having a higher curing speed and a highest curing state can be obtained, and the vulcanized products have an improved storability.
However, the afore-mentioned vulcanization systems are effective for acrylic rubber using vinyl chloroacetate as a cross-linking site but are still not satisfactory for the requirements for high strength materials, omission of secondary vulcanization and rapid vulcanization.